The determination of the stereochemistry and electronic structure of substrates in intermediate complexes of enzyme reactions is proposed by electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) techniques. Catalytically productive enzyme-substrate complexes will be stabilized by sub-zero enzymologic methods in single crystals for spectroscopic studies. The anisotropy of the ESR absorptions of paramagnetic substrates and of the ENDOR of protons in the vicinity of the paramagnetic sites of oriented single crystals will be analyzed to determine detailed stereochemical relationships of enzyme-substrate interactions and electronic perturbations in catalytically productive configurations. This information will permit the assessment of near transition-state configurations of enzymic catalytic residues and of substrates. The methods are designed to determine local structure of active site regions of enzymes and of substrates in productive binding modes to assess the structural and electronic basis of enzyme function on a detailed molecular level.